Atrial natriuretic peptide (hereinafter referred to as “ANP”), brain natriuretic peptide (hereinafter referred to as “BNP”), C-type natriuretic peptide (hereinafter referred to as “CNP”) and Dendroaspis natriuretic peptide (hereinafter referred to as “DNP”) are each members of a family of hormones known as “natriuretic peptides”. ANP and BNP share a wide spectrum of biological properties and belong to the cardiac natriuretic system. Both ANP and BNP are of myocardial cell origin while CNP is of endothelial cell origin. DNP was isolated from the venom of the green mamba snake and possesses structural similarity to ANP, BNP and CNP.
BNP received its name because it was first isolated from porcine brain, thus “BNP” stood for “brain natriuretic peptide”. However, because BNP belongs to the cardiac natriuretic system, “brain” has been changed to “B-type”. Therefore, “BNP” now refers to “B-type natriuretic peptide”.
ANP is secreted by the heart in the atria. BNP is secreted by the heart through the coronary sinus, predominantly from the cardiac ventricles. BNP is secreted as a 108 amino acid polypeptide precursor (See Valli et al., J. Lab. Clin. Med., 134(5):437-444 (November 1999)). The mature form of BNP is made up of 32 amino acids (representing amino acids 77-108 of the 108 amino acid polypeptide precursor) with a 17 amino acid ring closed by a disulfide bond between two cysteine residues, an amino-terminal tail of 9 amino acids, and a carboxyl-terminal tail of 6 amino acids. ANP and CNP also have a 17 amino acid ring closed by a disulfide bond between two cysteine residues. Eleven of the seventeen amino acids in the ring are conserved between the three molecules. In addition to the 17 amino acid ring structure, ANP has an amino-terminal tail of 6 amino acids and a carboxy-terminal tail of 5 amino acids. ANP is produced as a 126 amino acid pro-ANP form that is the major storage form of ANP. After proteolytic cleavage between amino acids 98 and 99, the mature 28 amino acid peptide ANP is found in coronary sinus plasma (See Yandle, J. Internal Med., 235:561-576 (1994)).
CNP is found in the brain and cerebral spinal fluid and is the most prevalent of the three peptides in the central nervous system. Little if any CNP is present in the heart. Pro-CNP is a 103 amino acid peptide that is processed into either CNP-53 (amino acids 51 to 103) or CNP-22 (amino acids 82 to 103) that are the active peptides. In addition the 17 amino acid ring structure, CNP-22 has an amino-terminal tail of 5 amino acids and contains no carboxy-terminal tail. CNP-53 is identical to CNP-22 except for a 31 amino acid extension at the amino terminal end.
As mentioned previously, DNP was isolated from the venom of the green mamba snake. The mature form of DNP is made up of 38 amino acids. DNP-like immunoreactivity (DNP-L1) has been reported in human plasma and the plasma concentration of DNP-L1 has been found to be elevated in patients with congestive heart failure (See, Cataliotti, et al., Mayo Clin. Proc., 76:111-1119 (2001)). Additionally, it is also known that the infusion of synthetic DNP results in marked natriuresis and diuresis in association with increased plasma and urinary cyclic guanosine monophosphate. Id.
One of the problems with natural human natriuretic peptides is that they are unstable in plasma and serum. Specifically, enzymes, such as proteases, cleave these peptides. For example, proteases cleave BNP (natural and synthetic) at various locations along its amino acid chain. For example, protease cleavage is known to occur at the amino terminus of BNP between amino acids 2-3 (Shimizu et al., Clinica Chimica Acta, 316:129-135 (2002)) and at its carboxy terminus between amino acids 30-32. Moreover, endopeptidase cleavage of BNP is also known in the art (Davidson and Struthers, J. Hypertension, 12:329-336 (1994)).
The measurement of mature BNP (i.e., the 32 amino acid molecule (amino acids 77-108 of the precursor polypeptide of BNP)) in humans (hereinafter referred to has “hBNP”), in the general population has been found to reflect cardiac diseases, such as congestive heart failure, ischemic heart diseases, atrial fibrillation and renal dysfunction. In fact, elevated levels of BNP in human plasma have been reported in heart disease, following acute myocardial infarction and during symptomless or subclinical ventricular dysfunction (See Mukoyama et al., J. Clin. Invest., 87:11402-11412 (1991), Motwani et al., Lancet, 341:1109-1113 (1993), Yoshibayashi et al., New Eng. J. Med., 327:434 (1992)). Increased circulating levels of ANP are seen in congestive heart failure, chronic renal failure and in severe hypertension. The presence of CNP in human plasma remains controversial with reports of its absence or presence as CNP-22 (See Yandle, J. Internal Med., 235:561-576 (1994)).
A ligand binding assay is an analytical technique for measuring concentrations of substances commonly referred to as ligands that react selectively with specific binding proteins. Immunoassays that measure the concentrations of antigens that react selectively with specific antibodies are an example of a class of ligand binding assays.
Ligand binding assays, such as immunoassays, for measuring human natriuretic peptides in plasma, particularly hBNP, are well-known in the art and are commercially available. These immunoassays require the use of at least one or two specific antibodies as well as at least one calibrator and, ideally, at least one control. In addition to the calibrators and controls, immunoassays require the use of at least one test sample. Test samples are normally biological samples derived from serum, plasma, whole blood or other bodily fluids (normally from a human patient). The levels of at least one human natriuretic peptide in the test sample is quantified in the immunoassay.
For example, U.S. Pat. No. 6,162,902 (hereinafter referred to as the “'902 patent”) discloses isolated antibodies that are monospecifically reactive to epitopes 1-10, 5-13 and 15-25 of hBNP. More particularly, the '902 patent describes two isolated monoclonal antibodies. The first monoclonal antibody is produced by hybridoma cell line 106.3 (A.T.C.C. Accession No. HB-12044) and is monospecifically reactive to epitopes 5-13 of hBNP. The second monoclonal antibody is produced by hybridoma cell line 201.3 (A.T.C.C. Accession No. HB 12045) and is monospecifically reactive to epitopes 1-10 of hBNP. The '902 patent also describes the use of the above antibodies in immunoassays for the purpose of quantifying the amount of hBNP in a biological sample. U.S. Pat. No. 6,677,124 (hereinafter referred to as the “'124 patent”) discloses a monoclonal antibody that binds to an epitope having the amino acid sequence of LYS-VAL-LEU-ARG-ARG-HIS (SEQ ID NO: 159) that is found in the C-terminal region of hBNP, namely epitopes 27-32. More particularly, the '124 patent describes a monoclonal antibody produced by hybridoma cell line BC203 (FERM BP-3515). The '124 patent also describes immunoassays for hBNP using this monoclonal antibody.
It is generally known in the art that the specificity and sensitivity of the antibodies used in immunoassays, such as hBNP immunoassays, are very important. One way in which to increase both the specificity and sensitivity of one or more antibodies is to improve the binding affinity of an antibody for its intended target (i.e., an antigen). Antibodies having an improved binding affinity for their intended targets should exhibit increased specificity and sensitivity. Therefore, there is a need in the art for new antibodies that specifically bind to human BNP with a high binding affinity and thus exhibit high specificity and sensitivity when used in said hBNP immunoassays.